1. Technical Field
The present invention generally relates to lighting systems, and more particularly, to light sources implementing light emitting diodes (LEDs).
2. Background
Many industries and applications need backlighting to illuminate an information source. In particular, transmissive liquid crystal displays (LCDs) have become very popular in many electronic media. LCDs are useful in applications such as, but not limited to, displays in avionics, laptop computers, video cameras, and automatic teller machine displays. However, many LCDs require backlighting to illuminate the information being displayed.
Many systems perform the backlighting function in conventional displays. For example, one way to backlight an information source employs an array of conventional straight tubular fluorescent lamps. While these conventional lamps are inexpensive and do not require complex electronic controls, they are sometimes inadequate for particular applications. For instance, in avionics applications, the poor color quality of the phosphors and the short lamp life of these conventional lamps, among other shortcomings, limit their usefulness.
To avoid the various problems with conventional lamps, many manufacturers employ customized lamps, such as tubular serpentine lamps. Unlike conventional fluorescent lamp arrays, custom-made serpentine lamps commonly provide good color characteristics, light luminance uniformity, and long lamp life. These lamps are typically hand made, and consequently, are comparatively costly. Moreover, these lamps. are extremely fragile and difficult to install. Therefore, while custom-made tubular serpentine lamps may meet certain standards for the backlighting function, the high cost and fragility associated with these lamps detract from the advantages they offer.
A third alternative for backlighting information sources is flat fluorescent lamps. An exemplary flat fluorescent lamp, described in U.S. Pat. No. 5,343,116, issued Aug. 30, 1994, to Winsor, comprises a substrate fritted to a transparent cover lid, forming an enclosure. Diffuse channels are formed into the substrate in the interior of the enclosure. Standard phosphors are added to the interior of the enclosure which is further flushed with a material for emitting energy, such as argon or mercury. Energy is emitted in the form of visible light when an electric potential is introduced to the lamp by two electrodes, with one electrode placed at each end of the diffuse channel. Such lamps potentially offer greater ruggedness and lower manufacturing costs than serpentine tubular lamp alternatives. However, these lamps are still costly to manufacture and are difficult to repair.
Yet another alternative for backlighting information sources implements LEDs. The use of LEDs as light sources can be advantageous for several reasons. LEDs have a long life, which reduces the frequency for replacing non-functioning diodes. Further, when it is time to replace an LED, replacement is easier and more cost effective than when replacing a fluorescent light source. Additionally, LEDs are mechanically robust, i.e., they can typically withstand greater shocks and vibration than conventional fluorescent lights. Referring now to FIGS. 1 and 2, a conventional light source 100 incorporating LEDs comprises an optical cavity 102, multiple LEDs 104, a power source (not shown), and a diffuser 106 (FIG. 2). Optical cavity 102 has a floor 108 in the interior portion of light source 100 and an exterior surface 110.
As shown in FIG. 2, in conventional LED light systems, the LEDs 104 are attached directly to the floor 108 of the optical cavity 102. Referring to FIG. 3, LED 104 typically comprises a surface mount device constructed by encasing a diode 300 near the center of a small translucent rectangular block 302. Electrical contacts 304 and 306 at the ends of block 302 connect to the diode via a small lead frame 308.
Conventional LED lighting systems, however, fail to perform adequately for many backlighting applications, such as avionics, in which strict display performance requirements restrict their use. For example, LEDs typically use power less efficiently than conventional fluorescent lamps to produce comparable light intensity. Further, a conventional fluorescent lamp relies on phosphors which have narrowly defined spectral emission peaks that must be carefully controlled to provide repeatable color output. Control of the phosphor mixture to produce production-quality lamps requires significant investment of time and effort to maintain a uniform mixture, produce an acceptable color point, and ensure color purity based on phosphor chemistry. Moreover, in conventional white LEDs, the spectral emission is dominated by the blue spectral emission, and thus, the resulting “white” light is heavily shifted toward the blue spectrum. This shift limits the usefulness of LED light sources in backlighting applications.